From d9515fb463185c46b053fa5ecbc716aa57f859ca Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 10 Oct 2024 18:52:05 -0700
Subject: [PATCH] Add a test to help track wasm features over time

This came up [on Zulip][zulip] where currently we have no notification
channel when the `generic` CPU for WebAssembly changes in LLVM other
than users hitting possible breakage. This is going to continue to
evolve over time, so the purpose of this test is to provide a location
to help indicate what to do when the `generic` CPU changes. The purpose
of this test is to provide a nudge to notify various folks that
something is changing and warrants release notes and such. Otherwise
it's expected that the test should be pretty easy to update if it starts
failing.

[zulip]: https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/Wasm.20minimal.20features.20target.20compiler-team.23791/near/476150388
---
 compiler/rustc_target/src/target_features.rs |  7 ++++
 tests/ui/wasm/default-enabled-features.rs    | 41 ++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 tests/ui/wasm/default-enabled-features.rs

diff --git a/compiler/rustc_target/src/target_features.rs b/compiler/rustc_target/src/target_features.rs
index e92366d5c5c3f..496f2e660abff 100644
--- a/compiler/rustc_target/src/target_features.rs
+++ b/compiler/rustc_target/src/target_features.rs
@@ -427,6 +427,13 @@ const WASM_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("sign-ext", Stable, &[]),
     ("simd128", Stable, &[]),
     // tidy-alphabetical-end
+    //
+    // Note that if a new feature is added above please add it to the test at
+    // tests/ui/wasm/default-enabled-features.rs to ensure that its
+    // on-by-default state is tracked over time. If you'd also be so kind as to
+    // update the reference, such as in
+    // https://github.com/rust-lang/reference/pull/1638, it would also be much
+    // appreciated.
 ];
 
 const BPF_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] =
diff --git a/tests/ui/wasm/default-enabled-features.rs b/tests/ui/wasm/default-enabled-features.rs
new file mode 100644
index 0000000000000..ec5c0edf49e00
--- /dev/null
+++ b/tests/ui/wasm/default-enabled-features.rs
@@ -0,0 +1,41 @@
+//@ only-wasm32
+//@ build-pass
+
+#![feature(wasm_target_feature)]
+
+fn main() {
+    #[cfg(any(
+        // on-by-default features
+        not(target_feature = "multivalue"),
+        not(target_feature = "mutable-globals"),
+        not(target_feature = "reference-types"),
+        not(target_feature = "sign-ext"),
+
+        // off-by-default features
+        target_feature = "atomics",
+        target_feature = "bulk-memory",
+        target_feature = "exception-handling",
+        target_feature = "extended-const",
+        target_feature = "nontrapping-fptoint",
+        target_feature = "simd128",
+        target_feature = "relaxed-simd",
+    ))]
+    compile_error!(
+        "\
+If this test fails to compile then it means that the default set of features
+active on WebAssembly targets is no longer what it used to be. This is likely
+due to LLVM being updated and changing the definition of the `generic` CPU in
+wasm.
+
+If you'd be so obliged please update the feature listings above this error
+message. Additionally please update this file too:
+
+* src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
+
+Specifically the section about \"Enabled WebAssembly features\". If you'd also
+be so kind as to ping wasm maintainers and/or the release team in the PR that
+updates this test it would also be much appreciated to ensure that this change
+is communicated out to users.
+"
+    );
+}